The present invention relates to a system for maintaining a playback head on a recorded track, and more particularly, to such a system that has playback speeds other than the recording speed.
The purpose of tracking systems, especially the present digital tracking system, is to develop a control voltage for, e.g., a bimorph crystal on which the playback head of the video tape machine is mounted, especially a helical scan video tape machine, so that even with abnormal playback speeds including standstill, i.e., other than the recording speed, the playback head stays on the video track. Before the system is described in detail, the basic requirements are explained in FIG. 1.
A track recorded on a portion of tape 10 is shown designated with the letter a. This recorded track has a certain angle .alpha. with respect to the lower edge of the tape. The angle is a result of two movements. One movement is the rotational movement of the head-wheel containing the head in the drum, and the other movement is the transverse movement of tape 10 pulled by the capstan motor while the drum is rotating. If the tape 10 is considered as standing still, the head in the drum would describe a line across the tape shown with the letter b. By moving the tape 10 from the right to the left side during the rotating head scan with the normal recording speed, the head exactly reads the recorded track a. If the tape 10 stands still and the drum with the reading head rotates, the head moves with respect to the tape according to line b. Therefore, to read out the recorded track when the tape is not moving, such as when it is desired to view a stop motion picture, the head has to be moved transverse with respect to the recorded track a in addition to its movement due to the drum rotation represented by b. In general, if the tape is moving at a tape speed V.sub.t, the head has to move with a velocity V.sub.h in the proper direction to scan the recorded track. The proper direction is against the tape movement as long as the tape speed is lower than the normal speed at which the tape was recorded. The head movement has to be in the direction of the tape movement as soon as the tape speed is higher than the normal tape speed at which the tape was recorded. The formula for the head speed required is V.sub.h =V.sub.o -V.sub.t, wherein V.sub.o is the normal recording tape speed and V.sub.t is the actual playback tape speed. This equation may be rewritten as V.sub.h /V.sub.o =1-V.sub.t /V.sub.o ; this is equal to 1-F/F.sub.o, wherein F is the number of fields scanned per second, which is proportional to the tape speed, and F.sub.o is the nominal number of fields scanned per second, which is about 60 per second in an NTSC system.
To make the head move with the velocity V.sub.h, one has to apply a ramp voltage waveform to the bimorph supporting the playback head. The slope of this ramp function must be 1-F/F.sub.o. It will be understood that for normal speed where F=F.sub.o, there is no need to apply a ramp waveform to the bimorph.
In existing systems this ramp waveform is generated using analog techniques, such as shown in U.S. Pat. No. 4,163,993. However, such a system is subject to component tolerances and power supply changes which result in unreliable operation. Further, during certan times, such as when the speed is changed or when the recorder is playing at normal speeds, it is desired to hold the ramp voltage constant. This is difficult with an analog system, which is subject to voltage drift. Still further, it is necessary to convert a tachometer digital output signal to an analog signal. The required converter is subject to failure. All of the above results in limited accuracy and stability.